ok this wont win any green points but an RTG generator core will heat a gang of nitanol bands and drive an electric dynamo, it will also be reasonably small enought to fit in a bird sized form factor, and will be thermally hot like a biological.

https://en.wikipedia.org/wiki/File:Radioisotope_thermoelectr...

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...